A plastic optical fiber (hereinafter “POF”) or a plastic clad fiber (hereinafter “PCF”) is proposed as an optical fiber which is used primarily in a short-range optical communication such as home LAN, office LAN, vehicle-mounted communication systems, and the like. Among them, both core and cladding of the POF are composed of plastic such as PMMA (polymethylmethacrylate) resin, and the like. The core of the PCF is composed of silica glass, and the cladding of the PCF is composed of polymer resin. The refractive index distribution of the core of those optical fibers is a step-type refractive index distribution in which the refractive index is constant within the core. As advantages of using the step-type refractive index distribution, it is possible to manufacture a core preform with a uniform refraction index at a low price, because it is possible to manufacture the core preform without adding additives such as germanium to the silica glass when manufacturing the core preform.
Patent Document 1 disclosed the POF. Patent Document 2 to Patent Document 4 disclosed the PCF.
The important parameters of an optical fiber include core diameter and numerical aperture (hereinafter “NA”) which are indicators of the ease of light-source coupling, as well as bandwidth which indicates transmission capacity. However, in the step-type optical fiber which is ordinarily composed of a single core and cladding, the NA and the bandwidth have an inverse relationship, such that it is necessary to narrow bandwith in order to enlarge NA, and reduce NA in order to widen bandwidth. When NA is reduced, the efficiency of coupling with a light source is lowered.
Ordinarily, the bandwidth of a multi-mode optical fiber primarily originates in intermodal dispersion. The intermodal dispersion is expressed by the group delay time difference (Differential Group Delay) between modes, and maximum intermodal dispersion is the difference between the mode where the group delay time difference is quickest and the mode where it is slowest (maximum group delay time difference). Generally, in the case of an one-step core structure which has a step-type refractive index distribution, the mode number is dependent only on NA or the relative refractive index difference (hereinafter Δ), and the quickest mode becomes the fundamental mode, and the slowest mode becomes the highest-order mode. Accordingly, in order to enhance bandwidth, it is necessary to reduce NA (or Δ) as mentioned above. However, as NA (or Δ) is not dependent on the core diameter, it is possible to select a desired core diameter without reference to the setting of NA (or Δ).
Generally, Δ and NA are expressed by the following formulae.
                    Δ        =                                            n              1              2                        -                          n              2              2                                            2            ⁢                          n              1              2                                                          (                  Formula          ⁢                                          ⁢          1                )                                NA        =                              n            1                    ⁢                                    2              ⁢              Δ                                                          (                  Formula          ⁢                                          ⁢          2                )            
Here, n1 represents the refractive index of the core, and n2 represents the refractive index of the cladding.
In order to solve such problems, the following has been proposed: to give the refractive index distribution of the core an α-power distribution (Patent Documents 5 and 6); multi-step core fiber composed of multiple cores with different refractive indices (Patent Document 7); two-step core fiber (Patent Document 8); and the like.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H08-122542
Patent Document 2: Japanese Patent Granted Publication No. 2794710
Patent Document 3: Japanese Patent Granted Publication No. 2836069
Patent Document 4: Japanese Patent Granted Publication No. 3446208
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. H08-304638
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2000-214342
Patent Document 7: Japanese Unexamined Patent Application, First Publication No. 2005-321686
Patent Document 8: Japanese Unexamined Patent Application, First Publication No. H10-282353